1. Field of the Invention
The invention relates to a liquid crystal display, and more particularly, to a liquid crystal display having uniform photo spacers and a method for fabricating the same.
2. Description of the Prior Art
Since liquid crystal displays (LCDs) have the advantages of portability, low power consumption, and low radiation, the LCDs have been widely used in various portable information products, such as notebooks, personal digital assistants (PDAs), video cameras, and etc. Furthermore, the LCD even has a potential to replace the CRT monitor or the television gradually.
In the conventional thin film transistor liquid crystal display (TFT-LCD) process, spacers, such as plastic beads, glass beads, or glass fibers, are positioned by spraying, and tend to be mal-distributed, for maintaining the thickness of the cell gap between two substrates of the TFT-LCD and controlling the cell gap to a specific value to ensure the display performance. Consequently, the contrast of the TFT-LCD is affected due to light scattering by the spacers that are positioned in the light transmitting regions, generating white point defects and reducing yield rates and the display performance. For this reason, photo spacers formed by a photolithographic process have been developed to replace the conventional plastic beads to control the dimensions and positions of the spacers and the uniformity of the cell gap accurately to raise the display performance.
Please refer to FIG. 1. FIG. 1 is a schematic diagram showing the structure of a liquid crystal display panel 10 according to the prior art. As shown in FIG. 1, the liquid crystal display panel 10 includes a lower substrate 22, an upper substrate 24, and a plurality of liquid crystals 26 filled between the lower substrate 22 and the upper substrate 24. The lower substrate 22 also includes a plurality of pixel electrodes 28 connecting to each thin film transistor (not shown) and an alignment layer 30, in which the pixel electrodes 28 are composed of transparent conductive material and the alignment layer 30 is disposed over the surface of the thin film transistors and the pixel electrodes 28 for controlling the initial arrangement of the liquid crystals 26. Additionally, the lower surface of the upper substrate 24 includes a plurality of black matrices 32, a plurality of color filters 34, a counter electrode 36 composed of a transparent conductive layer, and a plurality of photo spacers 38. Preferably, the color filters 34 comprise three kinds of color filters 34: red color filters 34a, green color filters 34b, and blue color filters 34c, in which the three kinds of color filters combine to form a pixel 20. The black matrices 32 are disposed on the border between each of the color filters 34 and the upper substrate 24. The counter electrode 36 covers over the surface of the color filters 34 to provide a voltage for the operation of the liquid crystal display panel 10. The photo spacers 38 are transparent or nontransparent minute columns, and they are used for replacing the spacers in the conventional liquid crystal display panel. The main consideration of the design of the distribution density of the photo spacers 38 is to arrange the photo spacers 38 uniformly in the whole plane of the liquid crystal display panel 10 to support the cell gap between the upper substrate 24 and the lower substrate 22, and at the same time adjust the height of the cell gap. The process of fabricating the liquid crystal display panel 10 according to the prior art involves forming the photo spacer 38 on the counter electrode 36 by a lithography process after depositing the counter electrode 36 on the bottom surface of the upper substrate 24. Additionally, the lower surface of the upper substrate 24 can further include an alignment film to cover the surface of the counter electrode 36.
By utilizing photo spacers, the liquid crystal display is able to obtain a much more uniform cell gap and much better penetration and contrast, thereby increasing the effectiveness of the liquid crystal display. Nevertheless, when the upper substrate and the lower substrate are assembled, the misalignment and shifting phenomenon often occur and results in uneven distribution of the photo spacers and influence the performance of the liquid crystal display. In other words, when a force is applied to the display panel, the cell gap will not be able to return to its original height as a result of insufficient distribution area and deformation of the photo spacers. Hence, Japanese Patent Nos. 2003-131238, 2003-207788, and 2003-84289 have disclosed numerous methods of forming photo spacers with different heights within the cell gap to solve the problem of insufficient distribution area and deformation of the photo spacers. However, the Japanese patents described above are still unable to effectively solve the shifting phenomenon problem when the upper substrate and the lower substrate are assembled. Moreover, two photomasks have to be utilized in the fabrication process to achieve the height difference effect within the cell gap, thereby increasing fabrication time and cost.